The invention relates to a service brake system for towed vehicles and particularly to a service brake system for double trailer combinations. It also particularly relates to a metering relay valve for use in such systems.
In known fluid-actuated service brake systems, the service braking is effected by pulses of fluid pressure induced in a service brake control line controlling a relay valve near the brake actuators. It is well known that because of the travel time of the fluid pressure pulses due to the distance between the towing vehicle or tractor and the towed vehicle or trailer, the brakes of the tractor may be applied prior to the application of the brakes in the towed vehicle. This problem is further compounded in a following second trailer or a trailing towed vehicle.
Various attempts have been made to increase the reaction speed to trailer braking. One solution has been the use of electrically operated valves which open or close by electrical command pulses. However, these known values require additional electrical wiring and control features which add complexity and greater chance for failure.
Another approach, taught by Stelzer in the U.S. Pat. No. 3,001,831 discloses an auxiliary valve interposed in the service brake line near the conventional emergency relay valve in order to increase the reaction speed of trailer braking. The valving action taught therein is that for a light pressure pulse in the service control line, the trailer braking pressure is to be brought to a point of nearly full braking pressure and then reduced ratio thereafter until at another point the delivered braking pressure is again finally approximately one to one with input pressure. While this valve reduced the delivery time of air pressure because of the amplification in the valve, there is no teaching of intermediate responsive control. Bray, in U.S. Pat. No. 4,090,738 teaches a booster air assembly including a relay valve having a closely coupled auxiliary air tank which may also be incorporated in a double trailer combination. The amplification of this valve is in one to one ratio up to a predetermined point where it increases to a ratio of 1.3 to 1. Speedup is predicated only upon the close proximity of the reservoir tank to the remainder of the system.